1. Field of the Invention
This invention relates to an X-ray imaging apparatus, and more particularly to an X-ray imaging apparatus wherein the amount of exposure of the object being imaged to the X-rays can be controlled by detecting the brightness of the image output by an image intensifier for converting an X-ray shadowgram into visible light.
2. Description of the Prior Art
In the control of the amount of exposure of an object such as a part of the human body to X-rays during X-ray imaging of the object, there are known techniques for detecting the brightness of the image output by the image intensifier constituting a part of the X-ray image intensifier optical system and used for converting the X-ray shadowgram into visible light.
In the conventional method for TV-imaging the X-ray image of an object, an image intensifier is disposed underneath the bed on which the object (patient) rests, a primary lens (primary lens group) is disposed at the output plane of the image intensifier, a beam splitter is disposed at an intermediate position between the primary lens and secondary lens groups, the light rays are passed through the beam splitter to a secondary lens (secondary lens group), and the light rays exiting from the secondary lens group are passed to a TV imaging device which produces a TV image of the object. A pick-up optical system is further disposed between the primary and secondary lens groups for measuring the amount of radiation transmitted through the patient resting on the bed, and the brightness of the image output by the image intensifier is controlled on the basis of measured value.
In the conventional arrangement, the pick-up optical system has ordinarily been disposed between the primary and secondary lens groups, as shown in FIG. 3. More specifically, a large diameter primary lens group 24 and a secondary lens group 25 (both groups being represented as single lenses in the drawing in the interest of simplicity) are disposed at a relatively long distance d from each other and a pick-up optical system 26 is disposed therebetween. The pick-up optical system 26 is constituted of a pick-up lens 26a made up of a lens and a prism, a field aperture 27, a condensing lens 26b and a photo-detector 26c. A part of the light of the image intensifier output image coming out of the primary lens group 24 is passed to the field aperture 27 by the pick-up lens 26a and forms an image at the field aperture 27. The field aperture 27 has an aperture which passes only a prescribed diameter portion of the image intensifier output image light and the light passing through the aperture of the field aperture 27 advances through the condensing lens 26b to the photo-detector 26c.
This arrangement has the following problems:
a) Since the primary and secondary lens groups have to be separated by a long distance (tandem interval), the size of the X-ray image intensifier optical system becomes large. Since the height of the bed above the floor is standardized within fairly narrow limits and cannot be increased substantially, any increase in the size of the X-ray image intensifier optical system is disadvantageous. Further, since shading (vignetting) occurs when the amount of peripheral light is insufficient, the diameter of the primary lens group has to be increased to secure an adequate amount of ambient light.
b) The positioning of the pick-up lens is critical and has to be very carefully selected so that a shadow of the pick-up lens does not appear in the TV image and degrade the X-ray image. Moreover, the occurrence of such a shadow cannot be avoided without using a primary lens of adequately large diameter.